Near field communications (NFC) devices are used for short range (typically of a few centimeters) transmission and reception of data. In a typical NFC interaction, an NFC reader transmits a signal that can be detected by an NFC tag. The NFC tag responds by generating a response signal that can be detected by the NFC reader
Typically the NFC reader is a powered device that generates a carrier signal onto which a data signal is modulated, and transmits the modulated carrier signal via an antenna of the NFC reader. This carrier signal is detected by an antenna of the NFC tag, which may be an active (i.e. powered) device, or may be a passive (i.e. unpowered) device.
NFC communications operate by magnetic coupling of the antenna of the NFC reader with the antenna of the NFC tag. When a signal is transmitted by the NFC reader, a magnetic field develops around its antenna, and this magnetic field couples to the antenna of the NFC tag, generating a voltage across the antenna of the tag.
A popular modulation scheme for NFC communications systems is known as 14443B amplitude modulation, in which the data signal to be transmitted is amplitude modulated onto the carrier signal generated by the NFC reader with a modulation depth as low as 10 percent. In the tag, the envelope of the amplitude modulated signal is detected to demodulate the data signal modulated onto the carrier. However, energy must also be harvested from the signal received by the tag, and the energy harvesting circuitry used in the tag has the effect of compressing the signal received by the tag, which effectively reduces the modulation present in the received signal, thereby making it difficult or even impossible to demodulate the data signal.
One possible solution to this problem is the addition of a high gain amplifier with automatic gain control to the NFC reader, to amplify the signal received by the receiver to a level at which the modulation present can be detected and demodulated to recover the transmitted data signal. However, it will be appreciated that this approach is limited by the degree of signal compression.